1. Field of the Invention
The present invention relates to a method of manufacture of a liquid crystal display panel. In particular, the invention relates to a method of manufacture whereby an operation of precisely mutually aligning the substrates of a liquid crystal display panel is simplified, such that the necessary assembly apparatus can be reduced in scale and complexity.
2. Description of the Related Art
In general, a liquid crystal display panel has the advantages of thinness, light weight, and low power consumption. For that reason, such panels are widely utilized in various types of electronic equipment, from pocket calculators to large-scale office automation equipment. The basic structure of a liquid crystal display panel is as shown in FIG. 1. A thin layer of liquid crystal 2 is retained between two substrates 16a, 16b by a sealing member 19. The substrate 16a has transparent electrodes 17a formed on regions of the inner surface thereof, with an orientation layer 18a (i.e. for molecular alignment of the liquid crystal) formed over the electrodes and the remainder of the inner surface of the substrate. The substrate 16b is similarly formed with transparent electrodes 17b and orientation layer 18b. Spacers 23 are disposed in the liquid crystal, for maintaining a uniform size of gap between the two opposing surfaces of the substrates.
In general, it is necessary to mutually laterally position the two substrates of a liquid crystal display panel to a very high degree of accuracy, i.e. to position one substrate very precisely above the other. The most generally used method of manufacture for such a liquid crystal display panel is as follows. Firstly, an empty cavity is formed, i.e. consisting of the two opposed substrates 16a, 16b accurately mutually aligned, and mutually attached by the sealing member 19 between them, but without the liquid crystal 2. The cavity is then filled with the liquid crystal 2, utilizing a vacuum insertion method. However such a method has various disadvantages, such as a considerable length of time being required to complete the process of filling the cavity with the liquid crystal 2, in the case of a large-size liquid crystal display panel.
For that reason, a method of manufacture has been proposed which is based upon first dropping liquid crystal onto a substrate, as described in Japanese Patent Laid-open No. 62-89025. That method is superior to the vacuum insertion method, since a shorter time is required to fill the space between the two substrates with the liquid crystal. The basic concepts of the "dropping" method are illustrated in FIG. 2, in which a sealing member 22 is formed in a peripheral region of one substrate 20a, while liquid crystal 21 is dropped onto the other substrate 20b. With the two substrates held spaced apart, the substrates are placed within a vacuum chamber of a vacuum assembly apparatus. In that condition (still under atmospheric pressure), the lateral positions of the two substrates 20a, 20b are mutually aligned, i.e. so that the substrate 20a becomes positioned precisely above the substrate 20b. The air pressure within the vacuum chamber is then reduced, and under the condition of low pressure, the two substrates are brought together so that the substrate 20a becomes superposed on the substrate 20b. Thereafter, the sealing member 22 is hardened, e.g. by application of suitable radiation.
However with such a prior art method, it is necessary to execute very accurate mutual lateral alignment of the two substrates while the substrates are within the vacuum chamber, with the substrates being held spaced apart by only a few millimeters. That spacing must then be accurately maintained while a condition of low ambient air pressure is established within the vacuum chamber, and then the two substrates must be accurately brought together such that one substrate is positioned exactly on top of the other. It is therefore necessary to achieve a positioning accuracy which is within the range of several microns to several tens of microns, for position adjustment and position control within the vacuum chamber. Hence, the overall size, complexity and cost of a vacuum assembly apparatus that utilizes such a vacuum chamber become excessive.
Furthermore it is found in practice that even if such a complex and expensive vacuum assembly apparatus is utilized, it is extremely difficult to attain a satisfactory degree of positioning accuracy.
Moreover with such a method, there is an increased possibility of dust or metallic particles being formed within the interior of the vacuum assembly apparatus. This will result in an increase in the most common causes of defects in liquid crystal display panels, i.e. defects due to the presence of dust particles within the panel cavity, short-circuits between the electrodes of the upper and lower substrates, etc.
For that reason, a proposal has been made to facilitate the substrate positioning, in Japanese Patent Laid-open No. 60-241020. With that method, position-determining pins are utilized for alignment purposes at the time of mutual attachment of the two substrates, with these pins having the same position relationships as position-determining pins which had been previously used during a process of forming the patterns of transparent electrodes on the substrates. That method has the advantage of enabling the vacuum assembly apparatus to be simplified to some extent. However the problem remains that it is impossible to achieve mutual positioning of the two substrates to within a sufficient degree of accuracy, i.e. to within several microns.